If the mesh terrain is loading slow, perhaps it's just your settings that are too high, so your system doesn't have enough spare cpu cycle to process it fast enough.
Adding a more complex vector product surely won't help, because the more complex the vector data is (more roads, coastlines, landclass varieties), the more terrain textures FSX will have to generate in real time.
If you stop the airplane mid-air, for example in Slew mode, and wait a bit, does it come correctly ?
If yes, that's exactly the reason above. Since the mesh terrain is handled by background threads, you might also try to experiment with different cpu affinity mask in the Add-on Manager tweaks. On a quad-core system, for example, 14 it's a good number. You will have to restart FSX after each change.